1. Field of the Invention
The present invention relates to an optical pickup capable of recording or reproducing information on or from an optical recording medium, and more particularly, to an optical pickup capable of reducing cross-talk by signal interference of adjacent tracks during playback of information from a main track.
2. Description of the Related Art
Recently, for high-density recording media, development of an optical pickup adopting a light source having a relatively short wavelength and an objective lens having a relatively large numerical aperture (NA), is increasing. For example, due to an increasing tendency toward use of a digital video disc (DVD), rather than a compact disc (CD), the configuration of an optical pickup has also changed. That is, development of an optical pickup adopting a light source having a wavelength of about 650 nm and an NA of 0.6, rather than an optical pickup adopting a light source having a wavelength of about 780 nm, i.e., infrared rays, and an NA of 0.45, has increased. By replacing the optical pickup, a high-density optical recording medium having a narrow track pitch can be adopted for information recording/playback.
As a track pitch which is the interval between adjacent tracks become narrow, there is concern about deterioration of a playback signal due to signal interference of the adjacent tracks. Such playback signal deterioration due to signal interference of adjacent tracks is referred to as xe2x80x9ccross-talkxe2x80x9d, the allowable range of which differs according to the type of optical recording medium. For instance, a DVD-ROM requires a cross-talk level of xe2x88x9230 dB or less.
As shown in FIG. 1, a conventional optical pickup capable of reducing cross-talk includes a light source 11, a grating 13 for diffractingly transmitting a light beam emitted from the light source 11 to produce 0th-order and xc2x11st-order diffracted beams, a beam splitter 15 for changing the traveling path of the beam incident thereto, an objective lens 16 for converging the incident light to form an optical spot on the optical recording medium 10, a photodetector 19 for receiving beams that have been reflected by the optical recording medium 10 and passed through the beam splitter 15, and a light receiving lens 19 arranged between the beam splitter 15 and the photodetector 19.
The 0th-order and the 1st-order diffracted beams, which have been diffracted by the grating 13, are simultaneously condensed as three optical spots S1, S2 and S3 on different positions of the optical recording medium 10, as shown in FIG. 2. That is, the 0th-order diffracted beam forms the optical spot S1 on a main track T1 from which an information signal is reproduced, while the 1st-order diffracted beams form the optical spots S2 and S3 on first and second adjacent tracks T2 and T3 adjacent at either side of the main track T1. For reference, the optical spots S1, S2 and S3 are formed beyond the corresponding tracks. That is, due to the narrow width of the tracks, the optical spots S1, S2 and S3 partially extend to the adjacent tracks.
Also, as shown in FIG. 2, the optical spots S1, S2 and S3 are formed on the optical recording medium 10 with a time lag. In other words, the optical spot S2 formed on the first adjacent track T2 precedes the optical spot S1 formed on the main track T1, and the optical spot S3 formed on the third adjacent track T3 is delayed relative to the optical spot S1.
The optical spots S1, S2 and S3 enter the photodetector 19 through the objective lens 16, the beam splitter 15 and the light receiving lens 17. As shown in FIG. 3, the photodetector 19 includes first through third light receiving portions A, B and C for respectively receiving the 0-th order and 1st-order diffracted beams, which have been reflected by the optical recording medium 10, and for photoelectrically converting the received 0-th order and 1st-order diffracted beams, respectively.
In the optical pickup having the above configuration, an information (radio frequency, RF) signal to be reproduced is reflected by the main track T1 and then received by the first light receiving portion A. Also, a part of the optical spot S1 of the 0-th order diffracted beam is formed on the first and second adjacent tracks T2 and T3, wherein RF signals of the first and second adjacent tracks T2 and T3, which are received by the first light receiving portion A, can be detected based on the RF signals of the second and third light receiving portions B and C, respectively.
In other words, the RF signal of the main track T1 is detected through operation with the signals detected from the first and second adjacent tracks T2 and T3, which is expressed by the following formula
RF signal=RF signal(first light receiving portion)xe2x88x92Kxc3x97[RF signal(second light receiving portion)+RF signal(third light receiving portion)]xe2x80x83xe2x80x83(1)
where K is an operation constant which minimizes jitter of the RF signal, that is, cross-tack due to signal interference of adjacent tracks.
In the optical pickup having the above configuration, the signals detected by the second and third light receiving portions B and C precedes or are delayed relative to the signal detected by the first light receiving portion A. That is, the signals of the first and second adjacent tracks T2 and T3, which are received by the first receiving portion A, are detected ahead or behind the detection of the optical spot S1 formed on the main track T1. Thus, it is basically impossible to operate the RF signals in real-time.
Meanwhile, as shown in FIG. 4, when an optical pickup is configured such that a time lag in forming the optical spots S1, S2xe2x80x2 and S3xe2x80x2 on the main track T1 and the first and second adjacent tracks T2 and T3, does not occur, as shown in FIG. 5, all three optical spots are received by the first light receiving portion A of the photodetector 19. In such a case, it is impossible to selectively detect the RF signal recorded on the main track T1.
Another conventional optical pickup capable of reducing cross-talk during playback due to signal interference of adjacent tracks is disclosed in Japanese Patent Publication No. Hei 6-150363 (dated May 31, 1994).
The disclosed optical pickup is characterized in that an optical spot formed on a main track and optical spots formed on adjacent tracks have a phase difference. As shown in FIG. 6, the optical pickup includes first and second light sources 21 and 22, a polarization beam splitter 24, a beam splitter 25, a phase plate 23 disposed between the second light source 22 and the polarization beam splitter 24, an objective lens 26, a polarization hologram optical element (HOE) 27, and a photodetector 28 for receiving beams that have been emitted from the first and second light sources 21 and 22 and reflected by an optical recording medium 20.
The first light source 21 emits a linearly polarized coherent light beam. The traveling path of the beam emitted from the first light source 21 is changed via the polarization beam splitter 24 and the beam splitter 25 toward the optical recording medium 20. The beam that has passed through the beam splitter 25 is converged by the objective lens 26 on the main track of the optical recording medium 20. The second light source 22 emits a linearly polarized coherent light beam having a polarization perpendicular to the direction of the beam from the first light source 21. The phase plate 23 transmits the incident beam from the second light source 22. The phase plate 23 is stepped with a different thickness d such that a transmission beam has a beam intensity distribution having at least two peaks at the center of the optical axis. In the optical pickup, the beam emitted from the first light source 21 is used as a primary beam, while that emitted from the second light source 22 is used as a secondary beam. The polarization beam splitter 24 transmits the beam from the first light source 22, and reflects the beam from the second light source 22, such that the beams head toward the optical recording medium 20. The polarization HOE 27 is disposed on the optical path between the beam splitter 25 and the photodetector 28, and selectively transmits the incident primary and secondary beams, which have been reflected by the optical recording medium 20. The photodetector 28 separately detects the intensity of the primary and secondary beams that have passed through the polarization HOE 27.
In the optical pickup having the above configuration, a polarized component of an optical signal read from the main track, and polarized components of optical signals from the adjacent tracks have a phase difference of 180xc2x0, so that the RF signal of the main track can be separated by the polarization HOE 27 without a need to provide a time lag in forming optical spots on the main track and the adjacent tracks. However, two optical spots, which are separated by using the phase plate 23, have a constant interval of 0.6 xcexcm therebetween. In other words, because the optical spot interval is not variable, cross-talk signals from adjacent tracks cannot be effectively removed when the track pitch is less than or greater than 0.3 xcexcm, even though the phase plate 23 is effective in reducing cross-talk during playback from an optical recording medium having a track pitch of about 0.3 xcexcm.
An object of the present invention is to provide an optical pickup capable of separating signals from a main track and first and second adjacent tracks during playback, in which an optical spot is converged on the main track, and other optical spots are converged across the first and second adjacent tracks, without a time lag relative to the optical spot formed on the main track.
Another object of the present invention is to provide an optical pickup capable of detecting a high-quality radio frequency (RF) signal, in which cross-talk can be reduced by operating a signal from a primary optical spot formed on a main track of an optical recording medium with signals from secondary optical spots formed on adjacent tracks.
According to an aspect of the object, the present invention provides an optical pickup comprising: a light source for emitting a light beam; a first light path changing means disposed between the light source and an optical recording medium, for changing the traveling path of an incident beam; a first polarization beam splitter disposed between the first light path changing means and the optical recording medium, for selectively transmitting or reflecting the incident beam according to polarized component to diverge first and second polarized component beams from the incident beam; a beam shaping unit disposed on the optical path of the second polarized component beam diverged by the first polarization beam splitter, the beam shaping unit for shaping the second polarized component beam; a second optical path changing means disposed on the optical path between the first polarization beam splitter and the optical recording medium, for making the first and second polarized component beams diverged by the first polarization beam splitter head in the same optical path; an objective lens disposed between the second optical path changing means and the optical recording medium, for converging the first and second polarized component beams incident thereto onto the optical recording medium; a second polarization beam splitter for making the first and second polarized component beams that have been reflected by the optical recording medium and incident thereto through the objective lens, the second optical path changing mean, the first polarization beam splitter and the first optical path changing means in sequence head in different optical paths; a photodetector having first and second light receiving portions for receiving the first and second polarized component beams, respectively, diverged by the second polarization beam splitter; and an operation unit for operating signals received by the first and second light receiving portions of the photodetector to eliminate cross-talk from a detected radio frequency (RF) signal.
According to another aspect of the object, the present invention provides an optical pickup comprising: a first optical module including a first light source for emitting a light beam, and a first photodetector for receiving the beam that has been emitted from the first light source and reflected by an optical recording medium; a second optical module including a second light source for emitting a light beam, and a second photodetector for receiving the beam that has been emitted from the second light source and reflected by the optical recording medium; a polarization beam splitter disposed between the first and second optical modules, and the optical recording medium, for selectively transmitting or deflecting the beam incident thereto according to polarization components, to change the traveling paths of the incident beams; an objective lens for condensing the beam incident thereto so as to form optical spots on a main track and first and second adjacent tracks of the optical recording medium; a transmission type phase difference prism disposed on the optical path between the second light source and the polarization beam splitter, for shaping the beam emitted from the second optical module to form an oval-shaped optical spot across the adjacent tracks of the main track, in a radial direction of the optical recording medium; and an operation unit for operating signals received by the first and second photodetectors to eliminate cross-talk from a detected ratio frequency (RF) signal.
In another embodiment, the present invention provides an optical pickup comprising: a light source for emitting a light beam; a beam splitter disposed between the light source and an optical recording medium, the beam splitter for changing the traveling path of the beam incident thereto; a first polarization beam splitter disposed between the beam splitter and the optical recording medium, for diverging first and second polarized component beams from the incident beam according to polarization directions, to reflect the first polarized component beam and to transmit the second polarized component beam; a reflection type phase difference prism for reflecting the second polarization component beam diverged by the first polarization beam splitter, and for shaping the beam incident thereto such that the beam reflected by the same is converged as optical spots on the adjacent tracks of a main track of the optical recording medium in a radial direction of the optical recording medium; an objective lens disposed between the polarization beam splitter and the optical recording medium, for condensing the first and second polarized component beams incident thereto on the optical recording medium; a second polarization beam splitter for directing the first and second polarized component beams incident thereto, which have been reflected by the optical recording medium and passed through the beam splitter, toward different optical paths; first and second photodetectors for receiving the first and second polarized component beams diverged by the second polarization beam splitter, respectively; and an operation unit for operating signals received by the first and second photodetectors, to eliminate cross-talk from a detected radio frequency (RF) signal.